Fix DER encoder implementations for output structures "EC" and "SM2"

Fix DER encoder implementations for output structures "EC" and "SM2"

These DER encoder implementations are supposed to be aliases for the
"type-specific" output structure, but were made different in so far
that they would output a "type specific" public key, which turns out
to be garbage (it called i2o_ECPublicKey()).  The "type-specific"
output structure doesn't support that, and shouldn't.

Fixes #16977

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16983)

(cherry picked from commit 2cb802e16fff3fb2c57ae664baa7bd9ce3e33805)